This invention generally pertains to fluid catalytic cracking (FCC) systems and processes, and more particularly to a system having a steam pressure let down integrated with a FCC unit, and further relates to recovering power from hot flue gas from an FCC catalyst regenerator.
FCC technology, now more than 50 years old, has undergone continuous improvement and remains the predominant source of gasoline production in many refineries. This gasoline, as well as lighter products, is formed as the result of cracking heavier (i.e., higher molecular weight), less valuable hydrocarbon feed stocks such as gas oil.
Refiners are more focused today than ever on improving utility consumption and reducing stack emissions. One area receiving significant interest is power recovery from the FCC flue gas, especially since this power is “clean” in that no additional CO2 is produced or emitted.
While much work has been done over the past 40 years to improve the reliability and operability of FCC flue gas power recovery systems, the process has remained largely unchanged; that is, until now. Traditionally, the FCC flue gas power recovery system has all too often been treated as an “accessory,” tacked on only to higher capacity, higher pressure FCC units in areas of high electrical cost. In order to make this technology useful for a wider range of FCC operators, innovative improvements have been developed to the way power recovery systems are incorporated into the FCC unit. These innovations significantly reduce the capital cost per unit of energy recovered from FCC unit flue gas in an environmentally friendly manner. These innovations can potentially double the return on investment (ROI) for a power recovery system when compared to traditional installations. This has greatly increased the application range of power recovery systems to FCC capacities for which it was previously considered uneconomical.
In its most general form, the FCC process comprises a reactor that is closely coupled with a regenerator, followed by downstream hydrocarbon product separation. Hydrocarbon feed contacts catalyst in the reactor to crack the hydrocarbons down to smaller molecular weight products. During this process, the catalyst tends to accumulate coke thereon, which is burned off in the regenerator.
The heat of combustion in the regenerator typically produces flue gas at temperatures of 677° to 788° C. (1250° to 1450° F.) and at a pressure range of 138 to 276 kPa (20 to 40 psig). Although the pressure is relatively low, the extremely high temperature, high volume of flue gas from the regenerator contains sufficient kinetic energy to warrant economic recovery.
To recover energy from a flue gas stream, flue gas may be fed to a power recovery unit. A power recovery train may include several devices, such as an expander turbine, a generator, an air blower, a gear reducer, and a steam turbine. The expander turbine may be coupled to a main air blower shaft to power the air blower of a regenerator of the FCC unit. In an expander turbine, the pressurized gas axially enters and radially exits the turbine. The kinetic energy of the flue gas is transferred through blades of the expander to a rotor coupled to a shaft to generate mechanical shaft power. The shaft may be coupled to either to a regenerator air blower, to produce combustion air for the regenerator, and/or to a generator to produce electrical power. Because of the pressure drop of 138 to 207 kPa (20 to 30 psi) across the expander turbine, the flue gas typically discharges with a temperature drop of approximately 125° to 167° C. (225 to 300° F.). The steam turbine may be included in the power recovery train for starting up or running the air blower for the regenerator.
Steam is produced for many purposes in a refinery. The flue gas from an FCC regenerator may be run to a steam generator for further energy recovery. Low pressure steam is typically generated at 241 to 448 kPa (gauge) (35 to 65 psig). Medium pressure steam is typically generated at 2413 to 3275 kPa (gauge) (350 to 475 psig) and high pressure steam is typically generated at greater than 4137 kPa (gauge) (600 psig). The various levels of steam generation can be accommodated through either box-style or shell and tube heat exchangers, but the box-style exchanger must be used if the flue gas is at lower pressure.
In order to reduce damage to components downstream of the regenerator, it is also known to remove flue gas solids. This is commonly accomplished with first and second stage separators, such as cyclones, located in the regenerator. Some systems also include a third stage separator (TSS) or even a fourth stage separator (FSS) to remove further fine particles, commonly referred to as “fines”.
The present invention provides a power recovery process comprising feeding steam to a turbine, letting down steam from a higher pressure to a lower pressure, generating power and directing steam exhaust from the turbine to an FCC unit.
One embodiment provides a power recovery system for use with an FCC unit, comprising a turbine for generating power by letting down steam from a higher pressure to a lower pressure for use in the FCC unit.
One embodiment provides a system for recovering power in an FCC unit, comprising a flue gas power recovery system including an expander; and a turbine for generating power by letting down steam from higher pressure to a lower pressure for use in a refinery process.
Additional features and advantages of the invention will be apparent from the description of the invention, figures and claims provided herein.